Coiled steel tubing, or simply “coiled tubing,” is single string of steel pipe that is continuously milled and coiled onto a large take-up reel for transportation and handling. It can be run into and out of a well bore at a high rate, relative to straight, jointed pipe, and, unlike wire line, it can be pushed into the well bore. It has been manufactured in lengths greater than 30,000 feet. Useful in a wide range of oilfield services and operations, including drilling, it is more often used after the well is drilled for logging, cleanouts, fracturing, cementing, fishing, completion and production related operations.
Coiled tubing is run in and out of well bores using a machine called a coiled tubing injector. The name “coiled tubing injector” derives from the fact that, in preexisting well bores, the tubing may need to be forced or “injected” into the well through a sliding seal to overcome the pressure of fluid within the well, until the weight of the tubing in the well exceeds the force produced by the pressure acting against the cross-sectional area of the pipe. However, once the weight of the tubing overcomes the pressure, the coiled tubing injector must hold it to prevent it from sliding further into the well bore.
When using coiled tubing to lower tools for performing operations relating to completing a well or working over existing wells, a blow out preventer (BOP) will be used for pressure control. The BOP is attached to the well head, and a coiled tubing injector is attached to the top of the BOP. In addition to a BOP one more of the following are can be attached to the top of a wellhead, through which the tools and coiled tubing may be lowered: a stripper, one or more sections of risers for accommodating one or more down hole tools for connection to coiled tubing prior to insertion into the well bore, a flow cross, and a backup BOP. The combination of a BOP with one or more other components is referred to as a BOP assembly. A BOP assembly may also be referred to as a pressure control stack. The exact components of the BOP assembly depends on the particular well, the work that needs to be performed on the well, and the down hole tools that will be used.
The BOP assembly can be assembled and transported to the well site. However, components of the BOP assembly—the BOP, the risers, and the stripper—are often transported to the well site for assembly. There also may be variation in the type of connectors used to connect the components of the BOP assembly. Generally, for well pressures up to and including 10,000 pounds per square inch (PSI) (68,948 kPa) quick connect fittings can be used to connect the components of the BOP assembly. However, where well pressures are expected above 10,000 PSI, connections between the components must be connected in a fashion to withstand these pressures. Connections between the various components of pressure control equipment, rated for service above 10,000 PSI, are required to be API ring bolted connections capable of withstanding the well head pressure. Connection of the various components of the BOP assembly could require making as many as six or seven of these connections, which involves fitting the seal rings and flange fasteners and then tightening them carefully.
When a BOP assembly is assembled at a well site, the assembly of the components, by the nature of their design, must be done vertically. People having to perform the work do so at elevated work stations. Additionally, the assembly of tools on the end of the coiled tubing must be done with the tubing running through the risers and BOP, below the bottom flange of the BOP, before the BOP assembly is connected to the wellhead. The components of the BOP assembly are typically suspended from a crane when the connections are being made. Usually this means that a coiled tubing injector is picked up first by a crane. After being picked up, the risers are connected to the coiled tubing injector, and then the BOP is connected to a bottom end of the risers. The entire assembly of injector, risers, and BOP will therefore be suspended from the crane. This is all done with one end of the coiled tubing from an adjacent reel inserted or “stabbed” into the coiled tubing injector. To attach the tools to the end of the coiled tubing, both the injector and BOP assembly are lifted by a crane high enough to connect the tools to the end of the coiled tubing that has been inserted through the risers and the bottom end of the BOP. The tools are sometimes very long. Installing and connecting the tools to the tubing is tedious and time consuming, usually involving more than one person.
Once the tools are connected to the coiled tubing, the entire assembly is then placed, by crane, on the wellhead and attached to the wellhead. After completion of the well intervention, cranes are again used to remove the BOP assembly from the wellhead and the coiled tubing injector and to dissemble the BOP assembly.